Method for increasing internal combustion engine exhaust gas catalyst durability

ABSTRACT

Durability of emissions catalysts in vehicular and other emissions control systems is significantly increased by positioning, prior to the catalyst-laden catalytic converter, at least one poison trap consisting of a low back pressure structure having a coating which reacts with non-metal or amphoteric oxides to form non-volatile reaction products.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention pertains to exhaust systems for internalcombustion (“IC”) engines, particularly to increasing the longevity ofcatalytic systems employed in IC engines as part of their emissionscontrol system.

[0003] 2. Background Art

[0004] Exhaust emissions from IC engines are regulated in virtually allindustrialized countries, with the aim of limiting emissions of unburntor partially burnt hydrocarbons (“HC”), nitrogen oxides (“NO_(X)”), andcarbon monoxide (“CO”). It was found early on that changes in such itemsas combustion chamber geometry, fuel/air flow paths, type of fuel, andair/fuel ratio all affect emissions. However, obtaining the mandatedemissions levels required far more emissions limitation than thesevariables could provide. Further, not only are some changes in theforegoing variables detrimental to power output and fuel economy, butmoreover, the effect of these variables on emissions is oftenconflicting. For example, increasing the air/fuel ratio generally lowersCO and HC, but increases NO_(x).

[0005] Thus, all present day automotive engines employ, in lieu of or inaddition to other measures, a catalytic system located in the exhaustpath. The principle component of such a system is typically termed a“catalytic converter,” and contains one or more catalytic elements whichlower HC, CO and/or NO_(x). Some catalyst systems are also designed toreduce particulate matter from diesel engines.

[0006] The individual catalysts may be coated onto ceramic or metalspheres, on metal screen or honeycomb, but most often are suppliedcoated onto a ceramic or metal honeycomb element termed a “monolith.”Also included in this category are monoliths applied as dieselparticulate filters (DPFs). A single converter or multiple convertersmay be used. The catalyst may be supplied either directly to themonolith as a component of a “washcoat” which is subsequently calcined,or may be deposited on a previously applied and calcined washcoat. Asmany as four or five monoliths may be placed in succession in theexhaust stream depending on the particular application. Emissionsrequirements have become increasingly stringent, requiring developmentof both new catalysts and higher catalyst loadings. In addition toabsolute emissions standards, emissions control system longevity, or“durability” requirements have also been extended. This maintenance ofoperation over extended periods has also challenged catalystdevelopment, and has required still further increased catalyst levels.

[0007] Catalyst durability is limited by numerous reactions which canoccur in the varied temperature and fuel/air stoichiometric environmentsin which the catalysts operate. For example, it was recognized quiteearly that lead, formerly supplied as an octane booster in fuel astetraethyl lead, is a serious catalyst poison. It thus has been removedfrom modern day fuels. However, numerous trace elements still come intocontact with the exhaust catalyst, some unavoidably so, and several ofthese are known to decrease catalyst durability. Not all these arederived from the fuel. For example, zinc dialkyldithiophosphates havebeen long used as antioxidants and/or high pressure lubricant additivesin motor oils. Especially with modern high speed engines, increasedpiston/wall clearances and decreased sealing allow increased entry ofoil into the combustion chamber, where oil additives, or theircombustion byproducts, subsequently pass into the exhaust stream.Products which are specifically problematic are acidic or amphotericoxides such as P_(x)O_(y), ZnO, and SO_(x). It has been proposed in Res.Disc. 39017, p. 650, October 1996, to trap acidic emissions products byincorporating basic metal oxides into the catalyst converter monolithwashcoat. However, the presence of additional components in theemissions catalyst washcoat has several notable drawbacks: first, itpartially obscures the active catalyst, thus requiring increasedcatalyst loading; second, the increased thickness of the washcoatincreases exhaust back pressure, lowering volumetric efficiency andengine power. Additionally, with some catalysts, the additional metaloxide components may react with catalyst components, altering theircatalytic activity.

[0008] Earlier, it was proposed in Japanese applications JP 55 151109and JP 56 044411, to insert an alumina-containing phosphorus trap in theoil recirculation system to remove suspect components from the oil beingrecirculated, and thus protect the exhaust catalyst. However, suchsystems are inefficient in the degree of protection achieved, may becomerapidly fouled, and may remove desirable antioxidant from the oil.

[0009] It would be desirable to provide a means whereby acidic catalystpoisons which lower emissions catalyst durability can be effectivelyremoved without requiring increased catalyst loading to compensate forreduced catalyst activity. It would be further desirable to increasecatalyst durability with little or no effect on engine exhaust backpressure.

SUMMARY OF THE INVENTION

[0010] It has now been surprisingly discovered that catalyst durabilitymay be significantly extended by positioning a flow-through element inthe exhaust gas path prior to the catalyst of the catalytic converter,this element being coated with a basic oxide which traps acidiccontaminants. By means of such devices, catalyst longevity may beincreased from 25% to 50% or more at low cost, without any significanteffect on power or fuel economy, and without requiring additionalcatalyst loading.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 illustrates one embodiment of a preferred tubular poisontrap in accordance with the present invention, and its preferredlocation in a catalytic converter;

[0012]FIG. 1a illustrates an enlarged view of a tubular poison trap inaccordance with the present invention;

[0013]FIG. 2 illustrates an embodiment of the present invention whereinthe poison trap is coated onto the interior of an engine manifold;

[0014]FIG. 3a illustrates a “monolith” poison trap element;

[0015]FIG. 3b illustrates the poison trap of FIG. 3a inserted into anexhaust pipe of an IC engine;

[0016]FIG. 4 illustrates a poison trap of enlarged cross-section locatedin a separate exhaust system canister;

[0017]FIGS. 5a and 5 b illustrate the positioning of the poison trapcomponents on the same first most monolith of the catalytic converter;and

[0018]FIG. 6 illustrates one embodiment of a conical poison trap inaccordance with the present invention.

[0019]FIG. 7 illustrates an embodiment of the present invention whereinthe poison trap is located in the engine manifold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The durability increasing devices of the present invention may betermed “poison traps.” These poison traps, and their active ingredient,contain a basic or amphoteric compound, preferably a metal oxide, whichis capable of reacting with gaseous acidic catalyst poisons to removethese from the exhaust stream. It is believed that the poison trapfunctions by reacting with volatile acidic contaminants to form asubstantially non-volatile compound which may be, without limitation, asalt, a mixed oxide, a ceramic material, or a glass. However, the exactmode of operation is irrelevant so long as catalyst durability isincreased.

[0021] The basic or amphoteric compounds must be capable of reactingwith and thereby trapping oxides of phosphorus, i.e., in the mannerdescribed above, and may be, without limitation, oxides of the metals ofGroups I and II of the Periodic Table of the Elements, or oxides(non-lniiting) of Ti, Zr, Mn, Fe, Co, Ni, Zn, Ga, Al, Si, Sn, Bi, Y, La,Ce, and/or Pr. Silicates, carbonates, and other compounds capable ofreacting with P_(x)O_(y) may also be used. In addition, compounds which,under high temperature operating conditions form such compounds, i.e.,are “precursors,” may be used as well. Many acetates and nitrates of thevarious metals are suitable precursors. A simple test may be applied todetermine whether any particular metal oxide, metal compound, or theirprecursors, including those not mentioned previously, are suitable. Insuch a test, hot exhaust gas from an IC engine, containing a minoramount of P_(x)O_(y) is passed through a poison trap employing theproposed metal oxide or metal compound. The poison trap may be followedby a catalytic converter. If the poison trap does not absorb ameasurable amount of P_(x)O_(y), or alternatively, if the catalyst lifeof the catalytic converter is not increased as compared to an identicalcatalyst tested under identical conditions but without the poison trap,then the metal oxide, metal compound, or precursor thereof is noteffective as a poison trap at the P_(x)O_(y) concentration and metaloxide or metal compound concentration used with that particularcatalyst.

[0022] The poison trap must be positioned in the exhaust stream prior tothe catalyst elements of the catalytic converter. The poison trap may belocated in the engine exhaust passages, more preferably in the exhaustmanifold, and most preferably in the exhaust header or in the catalyticconverter itself, prior to the emissions catalyst-laden substrate(monolith). By the term “exhaust valve or port” is meant the point ofexit of hot exhaust gases from the engine combustion chamber, i.e., theexhaust valve of a four stroke, valved engine, or the port of a twostroke engine.

[0023] The poison trap may constitute a tubular or conical poison trap,may constitute a coating on one of the exhaust components located priorto the exhaust catalyst itself, or may itself be a “monolith”,preferably one of relatively large cell size such that exhaust backpressure is not unacceptably increased. In particular, the exhaust backpressure should not be increased by more than 50%, preferably not morethan 25%, and most preferably 10% or less or unmeasurably, over theexhaust back pressure measured without the poison trap. Combinations ofsuch poison traps may be used. The poison trap, in general, is termedherein a “flow through element” which is inclusive of the various formsof the poison trap as disclosed herein.

[0024] Most preferably, the tubular poison trap is a HOT TUBE™, atrademark of Degussa, consisting of a hollow tube of ferrous metal,preferably stainless steel, having numerous holes in the wall thereof,and located appropriately in the exhaust stream and coated throughoutwith poison trap components. More than a single tubular poison trap maybe used, in parallel or in series with other tubular poison traps. Asuitable geometrical configuration of a tubular poison trap is describedin the assignee's U.S. Pat. No. 5,916,128 (Jun. 29, 1999), as shown inFIG. 1a herein. However, other configurations, such as conical-shaped,are possible. It is preferred that the geometry of the tubular poisontrap and any holes, slits, etc., in the tube impart some turbulence tothe exhaust stream to maximize contact of the hot gases with the metaloxides on the tubular poison trap's surface.

[0025] The preferred location of the tubular poison trap is in theconverter itself, prior to the first catalyst monolith. A preferredconfiguration is shown in FIG. 1, wherein the tubular poison trap issecured, preferably by welding, to a flange or an extension thereof atone end.

[0026] An alternative embodiment involves creating a poison trap of theexhaust manifold itself, by coating the interior passages of themanifold with a suitable poison trap washcoat, and preferably firing thewashcoat to bond it thoroughly to the manifold. Such an embodiment isshown in FIG. 2.

[0027] A further embodiment employs a ceramic or metal honeycomb. Inpreferred embodiments, the honeycomb has a cell size greater than thatemployed in a conventional monolith, such that little if any effect onback pressure is realized. Preferably, the cell density is about 20-400cells per square inch, and more preferably about 50-300 cells per squareinch. Such a monolith 30 is illustrated in FIG. 3a, and may be locatedin the exhaust manifold catalytic converter. Also, such a “monolith,” ifmetal, may be positioned directly in the exhaust stream as shown in FIG.3b, or may be located in a separate canister as disclosed in FIG. 4. Ifcontained in a separate canister, the cell size may be made smaller andthe overall diameter larger, to avoid increasing back pressure. Itshould be noted that the monolith 50 may be metal or ceramic whenlocated in a separate cannister.

[0028] The term “discrete” as used herein with respect to the durabilityenhancing flow-through element, means that the durability enhancingcoating is spaced apart from the first emissions catalyst monolith. Theactual location may be, for example, any of those previously described.In addition to those locations where the poison trap is physicallydistinct from the emissions catalyst, the poison trap ingredients mayalso be located on the same monolith as the first catalytic element, butpreceding the emissions catalyst. Such a monolith may be prepared bypositioning the majority of the active catalytic elements, generallyprecious metals such as platinum, rhodium, and/or palladium, on therearmost portion of the first monolith, with the frontmost portionserving as the poison trap. Such a monolith may be prepared by coating,preferably by washcoating, the monolith with a suitable poison trapwashcoat, followed by coating only the rearmost portion of the monolithwith a suitable emissions catalyst-containing washcoat.

[0029]FIG. 5a illustrates a coated monolith wherein the frontmostportion of the monolith 42 is coated with poison trap components 44,while the rearmost portion is coated with a conventional emissionscatalyst-containing washcoat 46. In this embodiment, components 44 and46 preferably abut each other or have a slight gap between them. Whilethis and other monoliths are shown to have the coating on the outersurface, it should be understood that the coatings could be, andpreferably are, on the inner, or cell surfaces, of the monolith, eitherin addition to, or in lieu of being on the outer surface.

[0030] In an alternative embodiment shown in FIG. 5b, the entiremonolith 52 is coated with a slurry containing the poison trapcomponents 54, dried and calcined. Next, a second coat containingcatalysts components 56 is applied over a rearmost portion of themonolith, and thus a rear most portion of the poison trap components, tocoat the rear most portion of the coated monolith for example, the rearhalf or two thirds of the coated monolith, with catalyst components. Inthis manner, the poison trap precedes the catalytic element as in otherembodiments. However, because these components are calcined prior toapplication of the catalyst-containing washcoat, they have little or noeffect on the emissions activity of the catalyst per se, except toprolong the life thereof.

[0031] The active ingredients of the poison trap are preferably appliedas a washcoat onto the supporting substrate, and subsequently calcined,or may be applied to a previously applied and calcined substrate. Anyconventional application procedure may be employed. For example, awashcoat of hydrophilic fumed silica, alumina, ceria, titania, ormixtures thereof, optionally with preferably, basic substances such asalkali metal hydroxides, alkaline earth hydroxides and the like may beapplied to the substrate and calcined. If the calcined washcoat does notalready contain the basic metal oxide or metal compound, then thecalcined washcoat may be dipped or sprayed with a solution or dispersion(if solid) of the metal compound, followed by drying and preferablycalcining. In some cases, the final calcination, or conversion ofprecursor to metal oxide or metal compound, etc., may take place insitu, i.e., when the element becomes exposed to hot exhaust gases.

[0032] In FIG. 1, the catalytic converter 1 comprises a can 7,preferably of stainless steel, that encloses multiple “bricks” or“monoliths” of emissions-catalyst laden ceramic honeycomb elements, forexample three elements 9, 11, and 13. The catalytic converter 1 isconnected to the engine exhaust stream at 3, by any convenient means,and to the tailpipe and any resonator, etc. present, at 15. Located inthe converter, preferably welded to a flange, is tubular poison trap 5,coated throughout with poison trap components. The location of tubularpoison trap 5 is prior to catalytic monoliths which might be poisoned byacidic vapors of the combustion process, particularly oxides ofphosphorous, and less preferably, sulfur. As best shown in FIG. 1a, thetubular poison trap 5 comprises a hollow cylinder 12 having the poisontrap components 17 coated thereon. The hollow cylinder 12 is formed of acylindrical wall 14 and includes a series of perforations 16 that arepreferably circular, oval, or the like in shape.

[0033] In an alternative embodiment illustrated in FIG. 6, a conicalpoison trap 60 having an essentially conical shape is shown secured,preferably by welding, to the catalytic converter or the exhaust pipe.The conical poison trap 60 comprises a conical member 22 having thepoison trap components coated throughout thereon. The conical member 22is formed of a conical wall and includes a series of perforations 16therein, and could comprise a typical flow distributor used in acatalytic converter. Though not shown for clarity, it should beunderstood that the surfaces that define the perforations 16 in FIGS. 1,1a and 6 are also coated with poison trap components.

[0034] In FIG. 2 is illustrated an engine exhaust manifold 20 having aport 21 through which exhaust gases pass, the manifold exhaust portbeing terminated by a flange 23 containing bolt holes 25 adapted toreceive a bolt to fasten the exhaust pipe to the engine manifold. On theinterior surface 24 of the manifold is coated the poison trap components27 of the present invention.

[0035]FIG. 3a illustrates a “monolith” poison trap element 30 ofrelatively large cross-sectional area cells 33. Preferably, to minimizeback pressure, element 30 has a cell density of 20-400 cells per squareinch, and more preferably about 50-300 cells per square inch. The poisontrap 30 may be secured to an exhaust header or pipe by conventionalmeans, such as by welding, as shown in FIG. 3b, where 40 is the internalcombustion engine, 41 the exhaust manifold, 43 the exhaust pipecontaining the monolithic poison trap 30, through which exhaust gasestravel to catalytic converter 45. While the poison trap 30 is shown inFIG. 3a as being generally cylindrical, it should be understood that thetrap 30 may have many different configurations, such as oval-shaped.While the poison trap 30 in FIG. 3a is shown as having the poison trapcomponents 34 coated on its outer surface for clarity, it should beunderstood that the poison trap components could be, and preferably are,coated on the cell surfaces, i.e., interior surfaces of the trap 30,either in addition to, or in lieu of, being coated on the outer surface.

[0036]FIG. 4 illustrates a poison trap 50 of enlarged cross-section,located in a separate canister 53 located between the exhaust manifold55 of the engine and the catalytic converter 57. The poison trap 50 maybe a monolith of small cell size due to its larger diameter, as theexhaust back pressure will be substantially unaffected. Distributionvanes 59 or other equivalent devices may aid in directing the gas flowover a greater portion of the flow through element 50.

[0037] In an alternative embodiment shown in FIG. 7, the poison trapcomprises a plurality of substrates 72, preferably metallic monoliths,that are coated with poison trap components and located in the exhaustmanifold 77 prior to the catalytic converter 79.

[0038] As the location of the poison trap is not critical, so long as itis located prior to catalytic elements which are susceptible topoisoning, it may conveniently take many forms. For example, a quiteturbulent location is on the vanes and/or interior housing of the drivenside of an exhaust turbocharger on engines so-equipped. It should benoted that if an emissions system contains a monolith coated with acatalyst system which is not subject to poisoning by acidic oxides, sucha monolith or catalytic element may precede the poison trap, the latterthen preceding one or more catalytic elements which are affected byacidic oxides.

[0039] By the term “flow through” is meant a portion of the exhaustsystem of an internal combustion engine downstream from the exhaustvalves or exhaust ports thereof, through which the exhaust of theinternal combustion engine passes. By the terms “exhaust catalyst” or“emissions catalyst” is meant a component which is designed to lower theamount of CO, HC, particulate matter and/or NO_(x), emitted by aninternal combustion engine, preferably exclusive of the “poison trap” ofthe present invention. By the terms “a” or “an” is meant “one or more”unless otherwise indicated.

[0040] Those skilled in the art are familiar with the emissionscatalysts and washcoats suitable for use with them. Reference may behad, for example, to U.S. Pat. Nos. 5,371,056; 5,610,117; and 6,103,660,which are incorporated herein for this purpose. These same washcoats,less all or substantially all the precious metal components, may be usedfor the poison trap washcoat. Additional poison trap components may beadded to the washcoat prior to or after deposition.

EXAMPLE 1

[0041] γ-aluminum oxide, stabilized with 2-4% by weight lanthanum(specific surface area 140 m²/g), is added to de-ionized water and themixture wet pulverized to form a slurry. The solids content is 45% byweight. A monolithic metal carrier possessing 400 cells per square inchis immersed in the slurry so as to completely wet it. Excess slurry isthen removed by blowing air through the carrier, after which it is dried(120° C., 2 h) and calcined in air (600° C., 2 h).

EXAMPLE 2

[0042] A coating dispersion is prepared by mixing lanthanum-stabilizedγ-aluminum oxide and barium acetate in a 10:1 weight ratio in de-ionizedwater. The solids content is 35% by weight. A hot tube™ of 6.5″ lengthand 1.5″ diameter is partially immersed in the slurry, such that theentire perforated portion of the tube is coated. Excess slurry isremoved by draining, after which the tube is dried and calcined asabove.

EXAMPLE 3

[0043] An aqueous coating dispersion is prepared by mixing de-ionizedwater with γ-aluminum oxide (specific surface area 180 m²/g) andtitanium dioxide (anatase, specific surface area 95 m²/g). The ratio ofaluminum oxide to titanium dioxide is 2:1 by weight and the solidscontent is 40%. The mixture is wet pulverized to form a slurry and thenused to coat a monolithic carrier by immersion. Excess slurry is allowedto drain from the carrier, after which it is dried in air (120° C., 1 h)and calcined (500° C., 2 h).

EXAMPLE 4

[0044] A carrier is coated with γ-aluminum oxide as described inExample 1. The dried and calcined carrier is impregnated with magnesiumby immersion in a 0.5 M aqueous solution of magnesium acetate, afterwhich the body is dried (120° C., 1 h) and calcined (500° C., 2 h).

EXAMPLE 5

[0045] A coating dispersion is prepared from lanthanum-stabilizedyaluminum oxide, barium acetate and de-ionized water as described inExample 2. A ceranic monolith possessing 600 cells per square inch ispartially immersed in the slurry, such that the front 1.5″ of themonolith (in the axial direction) is coated with the slurry. Afterdrying and calcining, a palladium-containing catalyst washcoat isapplied to the uncoated portion of the monolith by immersion of thatportion in a slurry prepared according to the example given in U.S. Pat.No. 6,103,660. The monolith is dried at 120° C. in air and calcined for2 hours at 600° C.

[0046] While the best mode for carrying out the invention has beendescribed in detail, those familiar with the art to which this inventionrelates will recognize various alternative designs and embodiments forpracticing the invention as defined by the following claims.

What is claimed is:
 1. A method of increasing the durability of anemissions control exhaust catalyst, said method comprising: positioninga low back pressure flow through element between the exhaust valve orport of an internal combustion engine and a catalytic converter element,said pass through element having deposited thereon a catalyst durabilityenhancing coating comprising a metal oxide, metal compound, a precursorthereof, said metal oxide or metal compound reactive with acidic gasescontained in an exhaust stream emanating from said internal combustionengine such that he durability of catalytic elements downstream fromsaid pass through element is enhanced.
 2. The method of claim 1, whereinsaid pass through element is a tubular poison trap, conical poison trap,grid, or honeycomb structure having deposited thereon a washcoatcomprising said metal oxide, metal compound, or precursor thereof. 3.The method of claim 1, wherein said pass through element comprises ahollow metal cylinder, the walls of said cylinder being perforated witha plurality of holes.
 4. The method of claim 1, wherein said metal oxideor metal compound is at least one of an oxide or silicate of one of Li,Na, K, Rb, Mg, Ca, Ba, Sr, Ti, Zr, Mn, Fe, Co, Ni, Zn, Ga, Al, Si, Sn,Bi, Y, La, Ce, or Pr.
 5. The method of claim 4, wherein said metal oxideor compound is produced in situ by passing exhaust gas through a passthrough element having a coating comprising a metal oxide or metalcompound precursor.
 6. The method of claim 1, wherein said catalystdurability enhancing coating is coated onto at least one of an engineexhaust manifold, an exhaust pipe, engine exhaust ports, or surfaces ofthe driven side of a turbocharger.
 7. The method of claim 1, whereinsaid catalyst durability enhancing coating is coated onto a firstportion of a monolith located in a catalytic converter, with a secondportion of said monolith being coated with an emissions reducingcatalyst.
 8. The method of claim 7, wherein there is no overlap betweenthe catalyst durability enhancing coating on said first portion of saidmonolith and said emissions reducing catalyst on said second portion ofsaid monolith.
 9. The method of claim 1, wherein said catalystdurability enhancing coating comprises a slurry of aluminum oxidecontaining a compound of barium, titanium, or magnesium, which issubsequently calcined.
 10. The method of claim 1, wherein said passthrough element comprising a monolith having at least a first portioncoated with a catalyst durability enhancing coating with a portion ofsaid first portion being coated with an emissions reducing catalyst,with said monolith being located in a catalytic converter.
 11. Anemissions control exhaust system for an internal combustion engine inwhich hot exhaust gases exit said engine from an exhaust valve or port,said exhaust system comprising a) at least one catalytic convertercontaining an exhaust catalyst which lowers at least one of CO, NO_(x),particulate matter or HC; b) positioned in the exhaust stream of saidengine prior to said exhaust catalyst, a discrete low back pressure flowthrough element having a catalyst durability enhancing coating thereon,said durability enhancing coating capable of removing at least a portionof acidic oxides contained in said exhaust stream.
 12. The exhaustsystem of claim 11, wherein said catalyst durability enhancing coatingis coated onto at least one of a) the engine exhaust passage locatedbetween the exhaust valve or port and the engine exhaust manifold; b)the exhaust manifold; c) an exhaust pipe; d) a separate canistercontaining a substrate on which said durability enhancing coating isdeposited; e) a tubular or conical poison trap, or f) the blades of aturbo charger fan.
 13. The exhaust system of claim 12, wherein saidpoison trap is welded to the interior of the leading end of saidcatalytic converter.
 14. The exhaust system of claim 11, wherein saidcatalyst durability enhancing coating comprises a slurry of aluminumoxide containing a compound of barium, titanium, or magnesium, which issubsequently calcined.
 15. An emissions control exhaust system for aninternal combustion engine in which hot exhaust gases exit said enginefrom an exhaust valve or port, said exhaust system comprising a) atleast one catalytic converter containing an exhaust catalyst whichlowers at least one of CO, NO_(x), particulate matter, or HC; b)positioned in the exhaust stream of said engine prior to said exhaustcatalyst, a low back pressure flow through element having a catalystdurability enhancing coating thereon, said element comprising anessentially exhaust emissions catalyst-free portion of a monolith, saidmonolith having an exhaust emissions catalyst located on a rear-mostportion thereof, said durability enhancing coating capable of removingat least a portion of acidic oxides contained in said exhaust stream.16. The exhaust system of claim 15, wherein there is no overlap betweenthe catalyst durability enhancing coating on said first portion of saidmonolith and said emissions reducing catalyst on said second portion ofsaid monolith.
 17. The exhaust system of claim 15, wherein said catalystdurability enhancing coating comprises a slurry of aluminum oxidecontaining a compound of barium, titanium, or magnesium, which issubsequently calcined.
 18. The exhaust system of claim 15, wherein saidcatalyst durability enhancing coating comprises lanthanium-stabilizedy-aluminum oxide.
 19. The method of claim 10 wherein said first portioncomprises the entire surface of said monolith.